The present invention refers to semiconductor lasers and, more specifically, to semiconductor laser structures including a substrate and an active laser layer, and to a method of making same.
Exemplary of a prior art semiconductor laser including a substrate and an active laser layer, is the arrangement shown in FIG. 1, which is currently referred to as a Semi-Insulating Buried Heterojunction (SIBH) structure.
Specifically, FIG. 1 is a cross section view including the mesa definition of an SIBH laser, including an n-type substrate 1 defining a mesa like structure laterally confined by an Fexe2x80x94InP semi-insulating layer 2. Multiple quantum well (MQW) active (i.e. lasing) layers 3 are covered by p layer 4, in turn covered by SiO2 mask 5. Finally, an n-InP layer 6 is superposed onto the Fexe2x80x94InP semi-insulating layers 2 and adjoins the sides of mask 5 as an anti-diffusion layer to prevent Znxe2x80x94Fe interdiffusion.
The structure described in the foregoing is conventional in the art and may be resorted to for manufacturing, i.e., SIBH-DFB (Distributed Feed Back) lasers operating e.g. in the 1.3 micrometer wavelength range.
High speed systems such as 10 Gbit/s Ethernet systems require uncooled laser sources capable of high temperature operation (above 80-90xc2x0 C.) as well as fast direct modulation behaviour. To achieve this, laser structures with low leakage currents together with low parasitics are strongly required.
Recently, devices with operating ranges extending above 100xc2x0 C. have been demonstrated by using multi-junction blocking layers.
The main disadvantage of these structures is related to their intrinsic high parasitic capacitance (hundreds of picofarads).
To reduce parasitic capacitance, fairly complicated structures including dielectric layers together with reduced contact area and narrow trenches (few microns away from the active stripe) are practically mandatory. Nevertheless, the minimum capacitance which may be achieved by resorting to these structures is in the range of 3 to 5 pF, which is still too high for the usual driver requirements.
Semi-insulating blocking layers (usually InP:Fe) are another possible solution, leading to a notable reduction of parasitics (capacitance values smaller than 1 pF have been demonstrated) and leakage currents at room temperature. A disadvantage of these prior art structures is leakage currents at high temperatures, due to the significant reduction in resistivity of the material with temperature; this may be about two orders of magnitude between 20 and 100xc2x0 C.
Also, from U.S. Pat. No. 5,825,047 an optical semiconductor device is known comprising a stripe-mesa structure provided on a semi-insulating substrate. The stripe-mesa structure comprises an undoped light absorption layer sandwiched by cladding layers and by burying layers on both sides. This structure aims at reducing device capacitance to provide wide bandwidth and ultra-high operation properties.
The need therefore exists for laser structures which are not unduly complicated and still offer the possibility of reducing both the leakage current and the parasitic capacitance, together with enhanced flexibility from the electrical point of view (e.g. number of bonding pads, directional bonding, uniform high speed injection over the active stripe).
An object of the present invention is to satisfy such a need.
According to one aspect of the present invention, such an object is achieved with a laser structure having an active region with at least one active layer, wherein the active region is in a ridge protruding from an exposed surface of a substrate carrying the region.
Another aspect of the invention relates to making such a laser structure by growing plural layers forming the active region including the at least one active layer over the substrate, and selectively removing at least part of the layers grown on the substrate to produce an exposed face and a ridge that protrudes from the exposed surface of said substrate, whereby said active region is included in the ridge.
As a result of the invention, leakage current is reduced by etching the laser structure to form the ridge that closely surrounds (about 10 micrometers away) the active region, thus reducing currents flowing in a lateral confinement layer. These currents are caused by recombination of carriers in the Fe-doped layer and by defects intrinsic to the technological process.
The reduced lateral area of the device thus obtained also leads to a reduction of parasitic capacitance, typically from 6 pF to less than 2 pF (as required by a typical IC driver), while providing bonding pads large enough for accommodating two 50 micrometers tape or wire bonding arrangements.
In the presently preferred embodiment of the invention, two bonding pads are longitudinally distributed or staggered along the ridge formation i.e. the active region or cavity of the laser device. Consequently, the laser structure is suitable for very high speed applications (in the 40 Gbit/s range), where modulation transit time plays a significant role.
Also, in the presently preferred embodiment of the invention, two possibilities are offered for wire bonding directions, resulting in maximum flexibility in module design. This aspect is significant for high speed applications, where the IC driver and the laser should be designed jointly to minimise parasitic inductance and capacitance in order to permit microwave operation of the module.